1. Field of the Invention
This invention relates to the explosive transfer between a booster and a warhead.
2. Description of the Related Art
The detonation of a warhead in a munition e.g., a missile, projectile, artillery shell, bomb, etc. is typically a multi-stage process to ensure both reliability and safety. It is important that the warhead detonate when triggered and not detonate accidentally due, for example, to mishandling or exposure to fire. The consequences of accidental detonation at a munitions depot or on-board a ship could be devastating. The explosive transfer between the booster and warhead can be a very challenging problem when trying to satisfy cost, interoperability, reliability and safety concerns.
As shown in FIGS. 1a and 1b, an exemplary munition 10 includes a warhead 12, a fuze 14 and a booster 16. Warhead 12 includes an external housing 18, a fuze well 20 formed from steel and lined with asphalt to provide insulation and protection from expansion/contraction, a charge tube fitting 22 for routing electrical cabling through the warhead to the fuze and explosive material 24 filling the warhead. In this particular embodiment, the fuze is not connected to electrical cabling through fitting 22 but the general purpose warhead includes the fitting nonetheless. Booster 16 includes an explosive pellet 26 that sits inside a housing 32. The booster is placed on top of the fuze, directly on top of the fuze's explosive pellet 34, and the assembly is inserted into the warhead's fuze well. A locking mechanism 36 secures the assembly.
To detonate the warhead, fuze 14 detonates its small explosive pellet 34, which transfers a pressure wave to the booster causing the booster explosive pellet 26 to detonate. Detonation of the booster generates a much larger pressure wave that is transferred to the warhead causing the warhead explosive 24 to detonate. In order to detonate the warhead explosive, the booster pressure wave that is transferred to the explosive material must exceed a characteristic ‘detonation threshold’ of the material. To address safety concerns, modern insensitive munitions (IM) compliant explosives are switching to explosive materials in the warhead that have a higher detonation threshold. The other factor that affects detonation transfer is the ‘barrier’ between the booster detonation and the warhead's explosive material. This barrier includes the steel fuze well and asphalt lining and the charge tube fitting that attenuate the pressure wave. The barrier also includes any airgap between the fuze well and explosive materials that will occur at low temperatures, which also attenuates the pressure wave. To ensure reliability, explosive transfer must be designed for the worst case conditions including thickness of the barriers and extreme cold.
FIGS. 2 and 3 depict a simulation of explosive transfer in which the warhead explosive failed to detonate because the booster pressure wave 40 (dynes/cm2) that was transferred to the explosive material 24 did not exceed the detonation threshold 42. In the depicted warhead design, the detonation threshold occurs at approximately 4.55×1010 dynes/cm2. As shown, the pressure wave emanates fairly uniformly from the booster explosive pellet 26 outward through the fuze well and into the warhead explosive material except that the wave is heavily attenuated at 90° from the booster interface by the charge tube fitting 22. Standard techniques to improve explosive transfer reliability include increasing the explosive energy of the booster. However, in the depicted simulation a 50% increase in explosive energy was still not sufficient to detonate the warhead. Furthermore, increasing the booster explosive increases costs, increases the total amount of explosive in a depot or on-board ship and may not fit in the available space in the fuze well in general purpose warheads with standard fuzes. For both economic and reliability reasons, the military places considerable demands on using proven general purpose warhead designs and fuze interchangeably in many different weapons systems. Another known technique that was tried was to include a ‘flyer plate’ on top of the booster. Upon booster detonation, the flyer plate is blasted forward so that its momentum into the explosive material triggers detonation. Unfortunately the charge tube fittings in a general purpose warhead lie directly in the path of the flyer plate rendering it ineffective.
By raising the detonation threshold to address accidental detonation, modern IM compliant explosives have made the task of reliable explosive transfer between the booster and warhead more difficult. A cost-effective solution for improving explosive transfer that can be used with general purpose warheads and fuzes is needed.